In the formation of color paper for photographs, ink jet images or digital images, it is known that the base paper has applied thereto a layer of polymer, typically polyethylene. This layer serves to provide waterproofing to the paper, as well as providing a smooth surface on which the imaging layers are formed. The formation of a suitably smooth surface is difficult requiring great care and expense to ensure proper laydown and cooling of the polyethylene layers. One defect in prior formation techniques is caused when an air bubble is trapped between the forming roller and the polyethylene which will form the surface for casting of photosensitive materials. This air bubble will form a pit that will cause a defect in the photographic performance of photographic materials formed on the polyethylene. It would be desirable if a more reliable and improved surface could be formed at less expense.
In color papers there is a need for providing color papers with improved resistance to curl. Present color papers will curl during development and storage. Such curl is thought to be caused by the different properties of the layers of the color paper as it is subjected to the developing and drying processes. This is especially true when the paper at the middle of a photographic element gains or loses water and pushes against stiffer plastic moisture insensitive layers bonded to the outside the paper. There are particular problems with color papers when they are subjected to extended high humidity storage such as at greater than 50% relative humidity. Extremely low humidity of less than 20% relative humidity also will cause photographic papers to curl. It would be desireable to provide a totally plastic composite base to improve internal stress differences that result in curl.
In photographic papers the polyethylene layer also serves as a carrier layer for titanium dioxide and other whitener materials as well as tint materials. It would be desirable if the colorant materials rather than being dispersed throughout the polyethylene layer could be concentrated nearer the surface of the layer where they would be more effective photographically.
While prior art photographic materials have been satisfactory, there is a need for images that can more closely replicate the actual scenes photographed.
One improvement would be sharpness, or the ability to replicate fine details of the image. This can be measured by mathematical calculations, one such method is called the MTF or Modulation Transfer Function. In this test, a fine repeating sinusoidal pattern of photographic density variation near the resolution of the human eye is exposed on a photographic print, when the print is developed the resulting density variation is compared to the expected density and a ratio is obtained to determine the magnitude of the transfer coefficient at that frequency. A number of 100 denotes perfect replication, and this number is relatively easy to obtain at spatial frequencies of 0.2 cycle/mm. At a finer spacing of 2.0 cycles/mm typical color photographic prints have a 70 rating or 70% replication.
Another improvement desired would be the visual appearance of whiteness in exposed subject areas like snow or a wedding gown. Because of imperfect light reflection from the surface underneath the image bearing emulsion, the current photographic prints tend to look yellow, and if corrections to the surface underneath the emulsion are made, then they may appear more gray or blue which is desired for some products. The measurement for this problem is a DMIN value which is a measurement of the photographic minimum density attained on a specially exposed print. In practice, it has been found that the surface under the silver halide layer can be measured to predict DMIN by using the L Star UVO value. The L Star UVO (ultraviolet filter out) can be obtained from a HUNTER spectrophotometer, CIE system, using procedure D65.
Improvements in another optical property affected by the base paper is opacity, or the ability of the photographic element to hide any visual evidence of what is behind the print. For example, the logo printed on the back, or the outline of the shadow of the fingers holding the print. Opacity numbers are generated by taking the ratio of the light reflected from the viewing surface of a generally white image when it is backed by a white background and then backed by a black background. A ratio of 1, which is reported as 100, is perfect. Most photographic materials today are rated at 92 to 95.
To improve optical properties, prior art photographic materials have suggested monolayer or coextruded layer coatings on raw paper base that are thicker and/or more concentrated with titanium dioxide (TiO.sub.2) and colorants.
Other high refractive index materials like zinc oxide or other finely divided solids are also used. In general, these improvements are costly. Processing and coating these concentrated layers can create manufacturing problems such as specks, lines and surface disruptions. The highly pigmented layers deteriorate the strength property of the coatings and may be involved with poor adhesion to the base paper or to the image bearing emulsion layer. Also, the coating speed of these layers may be lower. It would be particularly desirable if there was a way to produce improvements in MTF, LSTAR, and OPACITY at the same time without using levels of pigment loading that can cause manufacturing problems.
Another improvement would be to remove the need to maintain exact and even moisture profiles in the paper portion of imaging elements. During a critical hardening stage after emulsion coating, the interleaved layers of emulsion, plastic, and paper, wound in roll form, undergo a moisture exchange which affects the rate of the beneficial hardening chemical reaction. This exchange can be eliminated with an all plastic base, and as a result easier and cheaper methods for hardening can be used. This technology is well known and developed for use in the production of movie films, amatuer and professional film negatives, and x-ray products, all of which are coated with emulsion on totally plastic bases. It would be desirable if a suitable replacement for cellulose paper could be found for reflective imaging supports.
The assembly process of creating the photographic base materials is now at least a two step coating operation, the fiber paper base is made and then transferred to an extrusion machine for application of the plastic layers. The machine that produces the raw paper base is specialized for this purpose and often does not efficiently run the same speed and width as an extrusion laminating machine. It would be desireable to combine both operations into one cost effective, efficient, high, speed coating machine with less inventory.
It has been proposed in International Application Numbers PCT/US95/11222 and PCT/CA93/00385 to provide a paper like film comprising no paper fiber as a base that reproduces the feel and texture of paper in composite sheets. European Patent Application number 91307049.6 discloses a voided, coextruded, all plastic system that contains white pigments, antistat, and a paper-like printable surface that might be used as a paper replacement in photographic systems. As written, the applications all require more layers to be added before the composite imaging members are suitable for a base to be coated with photographic quality gelatin based emulsions.
It has been proposed in U.S. Pat. No. 5,244,861 to utilize biaxially oriented polypropylene in receiver sheets for thermal dye transfer.